ARSMALL 


UCTION  COILS 


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\ ^ 

INDUCTIUN  COILS 

How  to  Make  and  Use  Them. 


A Practical  Handbook  on  the  Construction  and  Use 
of  Medical  and  Spark  Coils . 


BY 


PERCIVAL  MARSHALL,  A.  I.  Mech.  E. 

'!/ 

Thoroughly  revised  and  enlarged  by 

KURT  STOYE, 

Instructor  Baron  de  Hirsch  Trade  School. 


EIGHTH  THOUSAND 


NEW  YORK 

SPON  & CHAMBERLAIN,  120  Liberty  Street 

1918 


(No.  6.) 


Copyright,  1906 
By  SPON  & CHAMBERLAIN. 


f 


CAMELOT  PRESS,  226*228  WILLIAM  ST.,  NEW  YORK,  U.  S.  A. 

1 ' 


a,  Ays*? 


v\ 


PREFACE  TO  REVISED  EDITION, 


In  revising  this  excellent  little  English  book,  one 
of  the  Model  Engineer  Series,  Mr.  Kurt  Stoye  found 
it  necessary  to  make  many  changes  especially  as  to 
gauges  and  prices;  to  add  additional  matter  in  nu- 
merous places  including  full  detailed  instructions 
for  making  a new  form  of  Electrolytic  Interrupter, 
with  illustration,  also  two  new  tables  specially 
compiled  for  this  work  showing  specifications  of 
materials  required  for  cods  giving  sparks  from  \ in. 
up  to  12  inches,  and  in  every  way  making  this 
work  a most  up-to-date  and  practical  American 
book. 


The  Publishers. 


► ' V* 


PREFACE, 


The  induction  coil  has  always  been  a popular 
piece  of  apparatus  with  amateurs  and  students 
interested  in  electrical  science,  so  numerous  and 
so  fascinating  are  the  experiments  which  can  be 
performed  with  its  aid.  Its  practical  importance 
has,  moreover,  been  considerably  enhanced  in  re- 
cent years  by  its  application  to  such  useful  pur- 
poses as  X-ray  work,  wireless  telegraphy,  and 
the  ignition  of  the  charge  in  internal  combustion 
engines,  such  as  the  petrol  motors  now  so  exten- 
sively used  for  cycles  and  automobiles.  In  the 
following  pages  we  have  adhered  to  the  lines 
usually  adopted  in  this  series  of  handbooks,  and 
have  commenced  by  giving  sufficient  theoretical 
information  to  enable  the  reader  to  understand 
the  various  principles  involved  in  the  design  and 
construction  of  satisfactory  apparatus.  This  is 
followed  by  descriptions  of  the  methods  of  build- 
ing cpils  for  various  purposes,  and  in  a later  chapter 
tables  of  dimensions  are  given  which  will  enable 
the  reader  to  construct  a coil  of  any  size,  within 
limits,  to  suit  his  own  special  requirements,  if 


VI 


PREFACE. 


these  are  not  met  by  the  coils  described  in  detail 
in  the  earlier  chapters.  The  concluding  section 
on  experiments  with  induction  coils  will  be  of 
interest,  as  showing  some  of  the  methods  by  which 
the  reader  can  derive  both  instruction  and  amuse- 
ment from  the  coil  he  has  made.  It  is,  of  course, 
impossible  within  the  limits  of  so  small  a book 
to  deal  exhaustively  with  this  portion  of  the 
subject,  but  the  hints  given  will  serve  to  direct 
the  attention  of  the  intelligent  student  to  many 
opportunities  for  carrying  out  attractive  experi- 
ments and  investigations  which  the  possession 
of  a well  made  coil  affords. 


PERCIVAL  MARSHALL. 


CONTENTS  OF  CHAPTERS 


PAGE 


I.  The  Principles  of  Electrical  Induction,  . . 1 

II.  Practical  Hints  on  the  Construction  of  Induc- 
tion Coils,  .......  8 

III.  How  to  Make  a Powerful  Shocking  Coil,  . . 1G 

IV.  How  to  Make  a J-in.  Sparking  Coil,  . . 25 

V.  The  Construction  of  a 4-in.  Sparking  Coil,  . 32 

VI.  How  to  Increase  the  Efficiency  of  Sparking  Coils,  . 48 

VII.  Particulars  of  Coils  for  Various  Lengths  of  Spark,  61 

VIII.  Experiments  with  the  Induction  Coil,  . . 64 


TABLES. 

I.  Approximate  Specifications  for  Spark  Coils,  # 62 

II.  Wire  Gauges  Compared  in  Decimal  Parts  of  an 

Inch, 63 


ABBREVIATIONS  USED  IN  BOOK. 
B.  & S.  = Brown  & Sharpe  wire  gauge. 

S.  C.  C.  = Single  cotton  covered  wire. 

D.  C.  C.  = Double  cotton  covered  wire. 

S.  S.  C.  = Single  silk  covered  wire. 

D.  S.  C.  = Double  silk  covered  wire. 


. 


CHAPTER  I 


The  Principles  of  Electrical  Induction. 

The  action  of  an  induction  coil  is  due  to  a prop- 
erty peculiar  to  electricity,  the  explanation  of 
which  is  entirely  unknown  to  science.  The  effect 
can  well  be  described  with  the  aid  of  a simple  dia- 


Fig.  1. 


gram,  as  in  Fig.  1.  Here  A B and  C D are  two  wires 
insulated  from,  but  parallel  to,  one  another. 
Suppose  the  ends  of  A B to  be  joined  to  the  poles 
of  a battery  or  other  source  of  electricity,  the 
galvanometer  G and  switch  or  contact  H being 
interposed  in  the  circuit.  The  ends  of  the  second 


2 


INDUCTION  COILS. 


wire,  C D,  are  to  be  similarly  joined  by  a wire 
which  has  only  the  galvanometer  F in  its  circuit. 
With  this  simple  apparatus  two  distinct  phenomena 
may  be  noted. 

The  switch  H being  ‘off’,  and  no  current  there- 
fore flowing  in  the  wire  A B,  the  needles  of  both 
galvonometers  will,  of  course,  stand  at  zero. 
Now  suppose  the  switch  H to  be  suddenly  moved 
to  the  ‘on’  position.  The  needle  of  galvanometer 
G will  move  to  another  position,  where  it  will 
remain  as  long  as  the  current  is  allowed  to  flow. 
It  will,  moreover,  be  found  that  the  needle  of  the 
second  galvanometer  F is  also  diverted  at  the 
moment  of  contact,  but  that  this  movement 
is  immediately  followed  by  a swing  back  to  the 
original  position  occupied,  namely,  to  the  zero 
point.  Obviously,  although  no  metallic  connection 
exists  between  the  wires  A B,  C D (these  being 
electrically  insulated  from  each  other),  a current 
has  momentarily  passed  in  the  circuit  of  which 
C D forms  a part,  but  has  very  quickly  ceased 
to  exist. 

The  switch  H is  now  to  be  moved  back  to  the 
W position.  The  movement  of  the  needle  of 
galvanometer  G is  back  to  the  zero  position,  but 
again  that  of  the  other  galvanometer  shows  a 
momentary  deflection — indicating  the  passage  of 
a current — and  this  time,  it  will  be  observed,  the 
movement  of  the  needle  of  F is  in  the  reverse  of 
its  previous  deflection,  indicating  the  important 
fact  that  the  currents  in  the  wire  C D are  in  opposite 


PRINCIPLES  OF  ELECTRICAL  INDUCTION. 


3 


directions  when  the  circuit  in  A B is  closed  and 
opened.  It  is  obvious  that  the  making  and  break- 
ing of  the  circuit  through  A B has  the  result  of 
‘inducing’  a current  in  C D, — how,  we  cannot  say, 
but  this  is  the  fact. 

By  means  of  suitable  apparatus  it  can  be  de- 
termined that  at  the  ‘make’  of  circuit  the  current 
induced  in  C D is  in  the  opposite  direction  to  that 
in  A B.  At  the  break  the  current  in  C D is  in 
the  same  direction  as  it  was  flowing  in  A B. 

Since  the  current  in  A B is  the  cause  of  the 
phenomena  just  described  it  is  called  the  primary 
current,  and  A B is  called  the  primary  wire.  The 
current  in  C D is  the  secondary — or  induced — 
current,  and  C D is  the  secondary  wire. 

Currents  are  induced  in  the  secondary  wire  only 
at  make  or  break  of  primary  circuit,  or  when  the 
current  in  the  latter  is  fluctuating  in  intensity. 
The  most  marked  results  are  observed  when  the 
make  or  break  is  sudden,  and  the  action  is  strong- 
est at  the  break  of  the  primary  current. 

The  inductive  effect  of  the  current  in  a wire  on 
another  wire  parallel  to  it  is  not  very  great  under 
the  conditions  already  given. 

Reverting  to  the  diagrammatic  circuits  shown 
in  Fig.  1.  If  we  imagine  the  wire  A B to  be  covered 
with  insulating  material  and  wound  on  a hollow 
bobbin  in  one  or  two  layers,  it  can  easily  be  seen 
that  the  secondary  wire  C D ean  be  wound  on  top 
of  it  to  any  desired  number  of  turns.  The  two 
sets  of  wires  will  still  be  parallel  throughout  their 


4 


INDUCTION  COILS. 


lengths,  and  the  whole  will  form  a compact  appara- 
tus. Indeed,  two  of  the  essential  parts  of  an 
‘induction  coil’  have  now  been  described,  and 
these  are  illustrated,  diagrammatically,  in  Fig.  2 
where  the  thick  line,  P,  encircling  the  tube  T,  is 
the  primary,  and  the  fine  lines  S,  two  layers  of 
the  secondary  wire.  It  does  not  matter  in  which 
direction  the  winding  is  done,  but  in  each  wire  it 
must  be  continuously  in  the  direction  in  which  it 
is  begun. 


Fig.  2. 


Although  the  coil  so  far  described  is  a more 
powerful  apparatus  than  Fig.l,  it  can  easily  be 
made  still  more  energetic.  If  the  tube  T,  which 
must  be  constructed  of  some  good  insulating 
material,  be  filled  with  an  iron  core,  the  inductive 
effect  of  the  coil  is  vastly  increased.  There  is  an 
intimate  connection  between  the  magnetic  lines 
of  force  and  the  inductive  action  of  electric  currents, 
and  the  introduction  of  the  magnetic  iron  core 
results  in  the  concentration  of  these  lines  of  force 


PRINCIPLES  OF  ELECTRICAL  INDUCTION. 


0 


into  a powerful  ‘field,’  surrounding  and  pene- 
trating both  the  sets  of  wires.  It  has  been 
found  that  the  highest  possible  effects  can  be  ob- 
tained from  any  given  induction  coil  only  when  the 
iron  core  is  of  the  softest  possible  iron.  This 
core  must  not  be  of  a solid  bar  of  iron,  but  must 
consist  of  a bundle  of  wires.  The  finer  these 
are  the  better;  the  tighter  they  are  packed  and 
the  closer  they  fill  their  allotted  space,  the  better 
will  be  the  result  of  the  coil’s  action. 

One  more  important  phenomena  must  be 
observed.  This  is  called  the  self-induction  of 
the  primary  wire.  It  could  not  be  discovered 
in  the  simple  apparatus  indicated  in  Fig.  1,  where 
the  primary  wire  (A  B)  consists  of  a single  strand 
only.  When  this  is  wound  into  a coil  as  at  P P, 
Fig.  2,  however,  it  is  found  that  every  single  turn 
acts  on  its  neighbors  just  as  though  they  were 
independent  coils  of  secondary  wire,  and  it  even 
induces  secondary  currents  in  them  in  the  same 
way.  This  is  very  important,  as  will  be  understood 
if  one  is  reminded  of  the  directions  taken  by  the 
induced  currents  at  make  and  break  of  circuit. 

It  will  be  remembered  that  at  the  instant  of 
closing  the  circuit  {starting  the  current  in  the 
primary)  the  direction  of  the  induced  current  in 
the  secondary  wire  was  in  the  opposite  direction 
to  that  in  the  primary  wire.  The  same  is  the 
case  with  the  current  induced  in  the  coils  of  the 
primary  wire  by  the  action  of  its  own  turns  of 
wire.  This  has  the  effect  of  weakening  the  current 


G 


INDUCTION  COILS. 


in  the  primary  wire,  since  it  is  impossible  for 
opposing  currents  to  traverse  the  same  wire  with- 
out affecting  one  another.  Incidentally,  it  may  be 
observed  that  the  very  weak  current  at  ‘make’ 
has  a correspondingly  weak  effect  on  the  true 
secondary  winding. 

Attention  should  now  be  turned  to  the  condition 
of  things  at  ‘ break  ’ of  circuit.  In  the  secondary 
coils,  a current  is  induced  similar,  as  regards 
direction,  to  the  inducing  current.  In  the  primary 
wire  the  same  effect  is  found,  and  since  the  two 
currents  (the  initial  and  the  induced)  travel  in 
the  same  direction  in  the  same  wire,  they  add  to- 
gether to  form  a powerful  rush  of  current  which 
results  in  a much  greater  effect  on  the  secondary 
coil. 

This  ‘ extra  ’ current  in  the  primary  wire  has 
the  effect  of  producing  a flashing,  bright  spark 
at  the  contact  breaker,  and  also  of  prolonging 
the  period  of  demagnetization  of  the  core.  This 
is  a great  disadvantage,  since  on  the  rapidity  of 
magnetization  and  demagnetization  depends  to 
a large  extent  the  power  of  the  coil.  To  reduce 
the  effect  of  the  ‘ extra  ’ current,  as  it  is  called, 
a condenser  is  introduced,  which  stores  the  rush 
of  current  until  the  circuit  is  again  ‘ made,’  when 
it  augments  the  battery  current  to  some  extent. 
Thus  the  principal  action  of  the  condenser  is  to 
reduce  the  destructive  sparking  at  the  contact- 
breaker,  and  to  shorten  the  period  of  demagneti- 
zation of  the  iron  core;  its  secondarv  effect  is  the 


PRINCIPLES  OF  ELECTRICAL  INDUCTION.  7 


great  increase  in  length,  thickness,  and  brilliance 
of  the  spark  from  the  secondary  coil.  Condensers 
for  coils  usually  consist  of  sheets  of  tin-foil  in- 
sulated from  one  another  by  sheets  of  paper 
soaked  in  paraffin  wax.  Their  construction  and 
method  of  connection  will  be  described  in  due 
course. 


CHAPTER  II 


Practical  Hints  on  the  Construction 
of  Induction  Coils. 

Induction  coils  may  be  divided  into  two 
distinct  classes — those  for  the  purpose  of  giving 
brilliant  and  powerful  sparks  for  experiment,  and 
those  used  solely  for  shocking  and  medical  pur- 
poses. It  is,  of  course,  possible  to  make  a coil 
serve  both  ends,  but  this  is  an  exceedingly  bad 
plan,  as  any  coil  capable  of  giving  even  quite  a 
small  spark  might  produce  serious  results  if  the 
whole  shock  were  taken  by  anyone  of  somewhat 
sensitive  nerves.  Generally,  coils  for  medical 
or  shocking  purposes  require  less  careful  work- 
manship than  the  others,  and  an  induction  coil 
of  this  character  is  most  suitable  for  the  amateur’s 
first  attempt  in  this  direction.  A sparking  coil 
must  be  constructed  with  scrupulous  regard  to 
insulation,  and  since  also  the  greatest  effect  is 
obtained  when  the  secondary  wire  is  closest  to 
the  primary,  practical  experience  is  essential  in 
its  making. 

Materials.  The  following  hints  on  the  ma- 
terials to  be  used  and  the  methods  of  connecting 
up  will  be  of  use  in  the  construction  of  a coil  of 

8 


CONSTRUCTION  OF  INDUCTION  COILS. 


9 


whatever  size.  The  various  parts  may  be  taken 
in  a definite  order,  commencing  with  the  core. 

The  core  of  an  induction  coil,  whatever  its  size 
or  purpose,  should  consist  of  soft  iron  wires.  They 
should  be  of  small  size,  Nos.  21,  22  and  23  B.  & S. 
gauge  being  usual  and  very  good  sizes.  This 
wire  is  bought  in  coils  and  can  be  cut  up  into 
pieces  the  required  length  after  the  wire  has  been 
straightened.  This  is  done  by  drawing  it  between 
two  rows  of  nails  fixed  in  a board,  as  in  Fig.  3,  so 
arranged  that  a very  slight  kink  is  given  to  the 
wire  as  it  passes  each  nail. 


Fig.  3. 


The  bundle  of  wires  having  been  thus  prepared, 
it  should  be  annealed  even  if  sojt  wire  has  been 
employed.  To  anneal,  place  the  bundle  in  the 
fire  the  last  thing  at  night,  so  that  the  wires 
become  thoroughly  red-hot  and  then  slowly  cool 
out  with  the  fire.  If  kept  red-hot  in  the  fire  for 
long,  the  thin  wire  will  oxidize  or  burn  away  and 
become  useless.  It  will  be  found  that  the  wires  have 
slightly  warped  during  the  annealing  process.  The 
wires  can  be  straightened  again  without  any 
trouble  by  simply  rolling  them  between  two 
hardwood  boards  3 or  4 at  a time.  The  further 
treatment  of  the  core  depends  upon  the  purpose 
of  the  coil,  and  may  therefore  be  left  for  the  present. 


10 


INDUCTION  COILS. 


The  primary  wire  in  ordinary  induction  coils  is 
always  thicker  than  that  employed  in  the  secondary 
winding,  and  both  primary  and  secondary  wires 
are  of  copper,  the  purer  the  better.  Primary 
wire  is  usually  double-cotton-covered  (D  C C), 
and  as  cotton  is  a somewhat  poor  insulator,  should 
always  be  soaked  in  melted  paraffin  wax  either 
before  or  after  winding  on  the  coil. 

The  secondary  wire  should  be  silk-covered  if 
the  greatest  possible  effect  is  to  be  obtained, 
as  a greater  length  of  wire  can  thus  be  wound  in 
a given  space.  It  is,  however,  very  expensive, 
and  ordinarily  its  use  may  be  confined  to  the  mak- 
ing of  large,  highly  finished,  and  expensive  spark- 
ing coils,  or  small,  neat,  medical  coils,  when,  if 
the  layers  of  wire  are  very  carefully  wTound,  it 
need  not  be  soaked  in  paraffin  wax,  a course  which 
is  essential  if  cotton-covered  wire  is  used  for  the 
secondary  winding. 

Sizes  of  wire.  As  to  the  sizes  of  wires,  the 
following  will  serve  as  a guide  (B.  & S.  gauge  is 
employed.)  For  shocking  or  medical  coils,  No.  20 
or  not  larger  than  No.  17  wire  maybe  used  for 
the  primary  winding.  For  sparking  coils  up  to 
b in.,  No.  16;  up  to  1 in.,  No.  14,  up  to  and  in- 
cluding 4 in.,  No.  12;  and  No.  10  for  larger  coils 
will  be  found  suitable  primary  wires.  The  second- 
ary wire  might  be  set  down  once  for  all  as  No.  36, 
this  wire  giving  with  good  workmanship,  nearly 


CONSTRUCTION  OF  INDUCTION  COILS. 


II 


l-in.  spark  for  every  pound  used,  and  being  of 
sufficient  sectional  area  to  produce  a thick,  snappy 
spark  well  suited  to  most  purposes.  If  a much 
thicker  spark  is  required,  at  a sacrifice  of  length, 
No.  34  wire  may  be  used;  similarly,  No.  38  or 
No  40  may  be  employed  to  produce  long,  thin 
sparks.  For  shocking  coils,  No.  32  or  34  is 
recommended. 

Paraffin  wax  is  used  almost  exclusively  nowa- 
days for  coil  insulation.  It  is  better  than  resin, 
shellac,  etc.,  and  is  practically  only  second-  to 
mica.  The  wax  should  be  hard,  clear,  pale  (not 
yellow),  and  perfectly  clean.  Pure  beeswax 
is  best,  but  is  much  more  expensive.  Great 
care  must  be  taken  when  melting  the  wax  not  to 
burn  it,  as  that  detracts  from  its  excellent  in- 
sulating properties.  The  best  plan  is  to  melt  it 
always  by  suspending  the  vessel  containing  it 
in  another  which  can  be  boiled — like  a glue  pot, 
in  fact.  The  wax  can  never  be  overheated  when 
that  method  is  adopted. 

Woods.  In  all  spark  coils  it  is  essential  to  have 
good  sound  construction,  and  the  wood  used  must 
be  perfectly  dry,  clear  grained,  and  carefully 
varnished  or  polished.  Ebonite  must  be  used 
in  many  parts,  especially  of  large  coils,  but  where 
‘ appearance  is  no  object/  good  sound  wood 
soaked  for  a long  time  in  molten  paraffin  wax 
will  be  found  a very  efficient  substitute.  Points 


12 


INDUCTION  COILS. 


must  be  rigorously  avoided  in  every  part  of  the 
apparatus  (except,  of  course,  the  discharging 
point  or  points). 

The  condenser  is  usually  fitted  in  the  base  of 
the  coil  in  a box-like  space  provided  for  it,  and  is 
unnecessary  for  a shocking  coil.  Its  position 
determines  the  dimensions  of  its  leaves.  It  is 
composed  .of  tin-foil  sheets  interleaved  with  larger 
sheets  of  waxed  paper.  In  the  smallest  sparking 
coils  there  should  be  a margin  of  \ in.  at  least 
all  around  the  tin-foil  sheet,  and  in  larger  sizes 
this  margin  should  be  proportionately  greater. 
In  a 6-in.  spark  coil  the  margin  should  not  be  less 
than  1^  ins. 

The  paper  for  the  condenser  should  be  moder- 
ately thin,  not  too  heavily  sized,  and  must  be 
flawless;  each  sheet  should  be  examined  in  a good 
light,  and  if  it  contains  the  minutest  pin-hole, 
or  any  thin  places,  must  be  rejected,  or  that  part 
at  any  rate,  must  not  be  used.  It  is  cut  to  suit- 
able sizes  and  soaked  for  a few  seconds  in  melted 
wax,  then  allowed  to  drain  and  cool  off.  The 
tin-foil  may  either  be  cut  oblong  and  have  separate 
connecting  lugs,  or  these  may  be  cut  out  all  in 
one  piece,  and  with  a little  planning  there  need 
not  be  much  waste  by  this  method.  It  has  the 
advantage  that  the  condenser  can  be  made  more 
compact. 

The  condenser  is  built  up  in  the  following  man- 
ner. A base  of  several  thicknesses  of  paraffined 


CONSTRUCTION  OF  INDUCTION  COILS. 


13 


paper  is  first  laid,  and  one  of  the  tin-foil  sheets 
placed  on  it  centrally,  as  in  Fig.  4 (A).  On  this 
is  placed  a single  paraffined  sheet,  and  on  this 
again  a second  tin-foil  sheet  with  the  lug  at  the 
other  end  (Fig.  4 B).  Next,  a paraffined  paper 


Fig.  4. 


and  a third  tin-foil  sheet  with  lug  as  in  A ; another 
paper,  and  a fourth  tin-foil  sheet  as  at  B.  This  is 
continued,  tin-foil  and  paper  alternating,  and 
alternative  tin-foil  sheets  with  lugs  at  opposite 
ends,  until  all  are  used  up.  A warm  iron  can  be 
used  to  press  down  the  condenser  and  make  it 
compact  and  secure.  All  the  lugs  at  each  end 


Fig.  5. 

can  then  be  clamped  by  means  of  a thin  piece  of 
springy  brass  to  which  is  soldered  a wire.  The 
usual  conventional  representation  of  a condenser 
is  as  Fig.  5,  the  paraffined  paper  being  omitted 
to  simplify  matters. 


14 


INDUCTION  COILS. 


Primary  Coil.  It  is  quite  possible  to  obtain  a 
strong  shock  from  a coil  consisting  of  a primary 
winding^  only,  with  the  usual  core  and  contact- 
breaker.  y A simple  form  is  shown  diagrammatic- 
ally  in  Fig.  6.  Here  the  core  is  represented  by 
C,  and  consists  of  a bundle  of  soft  iron  wires  about 
4 in.  long  and  f in.  in  diameter.  The  wire  wound 
on  this  core  may  be  about  four  layers  of  No.  24, 


Fig.  7. 

although  only  one  layer  is  represented  in  the 
drawing.  One  end  of  the  wire  is  joined  to  the 
support  of  the  contact  spring  S,  and  the  other 
end  to  a terminal  W.  Another  terminal,  X,  is 
joined  to  the  contact-screw  T.  A wire  is  taken  from 
T to  terminal  Y,  and  one  from  S to  terminal  Z. 
None  of  the  wires  may  touch  one  another  at  any 
part.  If  a battery  be  joined  to  terminals  W and 


CONSTRUCTION  OF  INDUCTION  COILS. 


15 


X,  and  a metal  handle  to  each  terminal  Y and  Z, 
a person  holding  the  two  handles  will  receive  a 
fairly  strong  shock.  The  effect  is  due  to  the  self- 
induction  of  the  coiled  wire,  as  already  explained. 
A condenser  must  be  not  used  with  such  a,  coil, 
as  it  will  absorb  the  ‘ extra  ’ current  which  is  the 
cause  of  the  shock. 

A diagram  showing  the  simplest  form  of  primary 
and  secondary  induction  coil,  without  a condenser 
is  shown  in  Fig.  7.  The  same  references  serve  as 
in  the  last  case,  but,  of  course,  we  now  have  the 
secondary  coil  (shown  by  the  fine  winding)  to  deal 
with.  Its  ends  are  simply  taken  to  the  terminals 
Y and  Z,  which  in  this  case  have  no  electrical 
communication  with  the  primary  current.  Fig.  8 
is  the  same  coil  with  the  condenser,  and  it  will  be 
seen  that  one  set  of  tin-foil  sheets  is  connected  to 
the  contact-spring,  and  the  other  to  the  contact- 
screw. 


CHAPTER  III 


How  to  Make  a Powerful  Shocking  Coil. 

A good  coil  for  shocking  or  medical  purposes 
should  be  easily  regulated  so  that  a current  of  any 
required  strength  may  be  administered.  This  can 
be  effected  in  various  ways,  but  the  best  method  is 
perhaps  the  regulation  by  sliding  the  secondary 
coil  on  or  off  the  primary  wire.  With  a coil  made 
as  follows,  this  is  easily  accomplished. 

The  baseboard  may  first  be  made.  It  should 
be  of  good  hard  wood — say  mahogany — planed 
perfectly  flat,  12  in.  long,  4^  in.  wide,  and  \ in. 
thick.  It  should  be  fitted  with  two  rabbeted 
pieces  of  wood  at  the  sides,  as  in  Fig.  10. 

The  core  is  next  to  be  made.  This  is  formed  of 
a bundle  of  soft  iron  wires  of  about  No.  22  gauge, 
4^  in.  long.  Select  a piece  of  thin  brass  tubing 
4^  in.  long  and  \ in.  in  diameter,  and  pack  this 
tightly  with  the  iron  wires.  Then  force  the  bundle 
out  an  inch  or  so  and  bind  the  protruding  end 
tightly  with  wire;  push  the  bundle  a little  farther 
and  continue  binding  it  and  pushing  it  out  until 
the  whole  bundle  has  been  secured.  The  ends 
may  then  be  filed  up  flat  and  dipped  in  soldering 

16 


HOW  TO  MAKE  A POWERFUL  SHOCKING  COIL.  17 

fluid.  The  ends  only  should  then  be  secured  by 
dipping  them  in  some  molten  solder  contained  in 
a ladle,  when  the  binding  wire  may  be  removed 
and  the  core  washed  in  water  to  get  rid  of  the 
remaining  soldering  fluid.  File  over  the  surface 
of  the  core  a little,  when  it  should  slide  freely  inside 
the  J-in.  brass  tube. 


Brass  tube.  Now  take  this  brass  tube  and  a 
sheet  of  stout  paper,  brown  paper  or  cartridge 
being  very  suitable.  Cut  the  paper  10  in.X  4 in. 
and  paste  it  with  good  flour  paste  or  mucilage 
until  it  is  thoroughly  soaked  and  quite  limp. 
Wrap  the  tube  with  two  turns  of  clean,  dry,  thin 
paper,  and  on  this  proceed  to  roll  the  pasted  strip, 
rubbing  it  down  well  all  the  time.  Allow  it  to 
dry,  when  the  brass  tube  and  the  paper  lining 
can  be  withdrawn,  and  the  result  should  be  a 
hard  paper  tube. 

Another  narrow  strip  of  paper,  £ in.  wide,  is  now 
to  be  wrapped  round  the  core  \ in.  from  its  end, 
and  is  to  be  well  pasted  or  glued  in  place.  It  must 
be  made  just  thick  enough  for  the  paper  tube  to 
fit  over  it,  and  if  made  too  large  in  the  first  place, 


18 


INDUCTION  COILS. 


can  be  shaved  or  filed  down  to  a suitable  size  when 
dry.  The  paper  tube  can  now  be  fixed  in  place  by 
glueing  it  on  the  collar,  making  it  flush.  The  brass 
tube  should  be  slipped  in  over  the  core  while  the 
joint  is  drying,  the  arrangement  being  shown  in 
Fig.  9 in  which  B is  the  brass  tube,  C the  core,  P the 
collar,  and  I the  outer  paper  tube.  When  dry, 
withdraw  the  brass  tube  and  soak  the  complete 
core  and  paper  tube  in  hot  paraffin  wax.  Then 
cut  a square  piece  of  mahogany  3 in.  each  way 
and  \ in.  thick,  find  the  centre  of  this  and  bore  a 
hole  just  large  enough  for  the  paper  tube  to  fit  in 
(at  the  collar  part).  At  the  top  of  the  wood  fit 
terminals  a and  6,  Fig.  11,  and  fit  the  core  in  the 
wooden  square  so  that  it  projects  by  the  amount 
left  outside  the  paper  tube,  namely,  J in.  Glue 
in  place  very  firmly,  the  arrangement  being  shown 
in  Fig.  11. 

The  primary  wire  is  to  be  wound  on  the  paper 
tube,  the  ends  being  joined  to  the  terminals  a,  6. 
About  4 or  5 oz.  No.  18  D.C.C.  wire  will  be  required, 
but  a little  more  should  be  provided,  and  it  should 
be  soaked  in  melted  wax  before  use.  Commence 
by  baring  one  end  of  the  wire,  scraping  it  clean 
and  bright,  and  twisting  it  round  under  the  terminal 
6,  which  may  then  be  screwed  down  tight.  Wind 
evenly  and  closely  nearly  to  the  other  end  of  the 
tube  and  then  return,  finishing  the  second  layer  at 
the  wood  end.  Again  wind  to  the  other  end  and 
back,  and  secure  the  remaining  end,  after  cleaning 


HOW  TO  MAKE  A POWERFUL  SHOCKING  COIL.  19 

it,  under  terminal  a;  the  primary  winding  is  now 
finished. 

The  core  and  primary  coil  may  now  be  mounted 
on  the  baseboard,  being  fitted  as  seen  in  Fig.  12, 


Fig.  13. 

the  square  wooden  cheek  being  glued  close  against 
the  rabbeted  sides  and  secured  by  a couple  of  wood 
screws  from  underneath.  Care  should  be  exercised 
in  fitting  to  get  the  core  nice  and  parallel  to  the 
edges  of  the  baseboard. 


20 


INDUCTION  COILS. 


The  secondary  portion  of  the  coil  now  demands 
consideration.  Cut  two  pieces  of  4-in.  hard  wood 
to  the  form  of  Fig.  13,  screwing  on  the  strips  at 
bottom  to  form  the  feet  which  are  to  slide  in  the 
guides  shown  in  Fig.  10.  Build  up  a strong  paper 
tube  in  the  manner  already  indicated  when  the 
primary  coil  was  under  discussion.  This  paper 
tube  is  to  be  of  such  diameter  inside  that  it  will 
slide  freely  over  the  finished  primary  wire;  the 
inside  diameter  should  not  be  more  than  ^ in. 
larger  and  the  thickness  should  be  about  three 
layers  of  cartridge  paper;  it  must  be  thoroughly 
well  pasted  or  glued.  The  length  is  4 in. 

Take  the  diameter  of  the  tube  when  dry  and 
make  holes  just  big  enough  to  take  it  in  the  middle 
of  each  wooden  cheek  as  at  x Fig.  13.  Glue  the 
cheeks  firmly  and  squarely  at  each  end  of  the  tube 
and  set  aside  to  dry.  Afterwards  fit  a terminal  at 
the  top  of  each  cheek  when  the  bobbin  way  be 
wound  full  of  No.  34  D.  C.  C.  wire.  Before  actually 
starting  this  operation  try  the  bobbin  in  the  slide 
to  see  that  the  feet  not  only  run  freely  in  the 
grooves,  but  also  that  the  paper  tube  clears  the 
primary  winding  also. 

A simple  winding  machine  will  be  needed  if  the 
reader  cannot  make  use  of  a lathe  of  any  sort. 
This  winder  need  only  consist  of  two  uprights  of 
wood  fixed  on  a simple  wooden  base  as  in  Fig.  14. 
The  bobbin  of  wire,  which  should  have  been 
soaked  in  melted  paraffin  wax,  is  supported  by  an 


HOW  TO  MAKE  A POWERFUL  SHOCKING  COIL.  21 


axle  consisting  of  a piece  of  stout  wire.  The 
secondary  bobbin  is  mounted  on  a wooden  core, 
which  is  also  supported  firmly  on  a wire  axle, 
one  end  of  which  is  cranked  to  form  a handle. 
The  wire  should  be  run  through  the  hand,  a fairly 
long  ‘ bight  ’ being  taken  as  seen  at  M,  and  an 
old  glove  should  be  worn  during  the  operation. 
The  beginning  of  the  wire  is  to  be  cleaned  and 


Fig.  14. 


passed  through  a tiny  hole  in  the  cheek  and  se- 
cured under  one  terminal,  and  to  save  possible  dis- 
appointment it  should  be  tested  by  means  of  a 
galvanometer  and  battery  before  beginning  to 
wind. 

Great  care  should  be  taken  to  wind  evenly  and 
closely  from  one  end  to  the  other.  When  one 
layer  is  finished  it  must  be  covered  with  a layer 
of  thin  waxed  paper,  and  the  second  laver  wound 


22 


INDUCTION  COILS. 


back  again  to  the  first  end,  always  turning  the 
handle  H Fig.  14  in  the  same  direction.  Should 
the  winding  of  the  second  layer  draw  the  waxed 
paper  away  from  the  wooden  cheek  a narrow 
strip  of  the  same  paper  should  be  laid  on  the  un- 
covered wire  (See  Fig.  18),  as  it  is  at  the  ends 
that  there  is  the  greatest  tendency  to  spark  from 
one  layer  to  another.  For  this  reason  every  care 
must  be  exercised  to  prevent  the  last  turn  in  any 
layer  from  sinking  down  into  any  space  against 
the  cheeks.  When  all  the  wire  is  wound  on,  or  the 
coil  has  reached  within  \ in.  of  the  edges  of  the 
bobbin  ends,  the  last  end  of  the  wire  should  be 
bared  and  secured  under  the  second  terminal  The 
last  layer  of  wire  should  finish  at  that  end ; in  other 
words,  there  must  be  an  odd  number  of  layers. 

The  coil  may  be  covered  with  velvet,  silk,  thin 
‘ paper/  ebonite,  or  may  be  left  bare  as  the  maker 
chooses.  Similarly  the  finish  of  the  wood-work  is 
left  to  his  own  taste. 

Handles  should  be  made  of  thin  brass  tube 
(preferably  nickel-plated)  about  4 in.  or  4 \ in. 
long  and  f in  or  more  in  diameter.  They  are 
joined  to  the  secondary  terminals  by  means  of 
flexible  silk-covered  copper  wires  the  ends  of 
which  are  soldered  to  brass  connectors. 

The  contact  breaker  is  shown  at  Fig.  15.  C is 
an  ordinary  bell  contact  screw  pillar,  the  screw 
itself  being  tipped  with  platinum  W.  The  brass 
spring  H is  of  suitable  height  to  carrv  the  iron 


HOW  TO  MAKE  A POWERFUL  SHOCKING  COIL.  23 


armature  A exactly  opposite  the  core  end.  The 
lower  end  of  the  spring  is  soldered  to  a stout  bar 
of  brass  B,  to  which  also  is  soldered  the  small 
terminal  R.  A stout  piece  of  platinum  foil,  P, 
is  soldered  on  the  spring  just  where  W meets  it. 
The  connections  of  the  coil  are  those  shown  in  the 
last  chapter  at  Fig.  7,  the  primary  terminals  W 


and  X in  that  illustration  being  here  connected 
one  to  one  terminal  of  the  primary  winding  (a  Fig. 
11)  the  other  to  C Fig.  15;  terminal  b Fig.  11 
being  also  joined  to  R in  Fig.  15.  For  appearance 
sake  connections  should  be  effected  as  far  as  pos- 
sible by  wires  carried  in  grooves  underneath  the 
baseboard. 


24 


INDUCTION  COILS. 


Regulation  in  this  coil  is  effected  first  by  sliding 
the  secondary  coil  on  or  off  the  primary  coil  and 
secondly  by  withdrawing  the  brass  tube  from  the 
core.  To  do  this  easily  its  end  should  be  fitted 
with  a tight  plug  of  wood  turned  to  the  form  of  a 
little  handle  as  in  Fig  9. 

The  full  effect  of  this  coil  is  obtainable  when 
two  small  bichromate  cells  are  employed,  and  it 
should  then  prove  very  powerful  indeed.  One  cell 
will  ordinarily  serve,  and  even  two  dry  cells  will 
give  good  results  if  not  worked  for  too  long  a time, 


CHAPTER  IV. 


How  to  Make  a |-in.  Sparking  Coil. 

The  amateur  who  has  successfully  built  a shock- 
ing coil,  and  has  thus  gained  an  insight  into  the 
difficulties  and  principles  of  coil  construction, 
will  find  a ^-in.  sparking  coil  an  excellent  test  of 
the  skill  he  has  acquired.  To  those  more  advanced 
in  such  work  a coil  of  this  size  may  be  commended 
as  one  of  practical  utility,  a special  application 
being  the  ignition  of  the  charge  in  a gas  or  oil 
engine,  or  for  motor  car  purposes. 

Materials  required.  The  principal  dimensions 
can  be  taken  from  Chapter  VII.,  from  which  it 
will  be  seen  that  the  core  is  to  consist  of  a bundle 
of  iron  wires — the  softer  and  finer  the  better — 6 in. 
long,  5x8  in.  diameter.  This  should  be  made 
very  compact  and  tight,  and  should  be  wrapped 
from  end  to  end  with  a layer  of  tape,  being  secured 
at  the  finishing  end  by  stitching.  On  this  is  to 
be  wound — very  tightly  and  closely — two  layers 
of  primary  wire  No.  16  D.C.C.,  leaving  -|in.  of  the 
core  at  each  end  uncovered.  The  simplest  way  of 
securing  the  ends  of  the  primary  wire  is  to  bend 
each  along  in  the  direction  of  the  core,  but  on 


25 


26 


INDUCTION  COILS. 


opposite  sides  of  it,  securing  by  means  of  strong 
cotton  tied  across,  as  in  Fig.  16. 

Insulation.  The  importance  of  sound  insula- 
tion between  primary  and  secondary  winding  can- 
not be  overrated,  but  it  can  be  easily  ensured. 
To  do  this,  cut  a strip  of  calico  or  linen  about  5 in. 
wide  and  30  in.  long  This  is  to  be  wound  evenly 


and  rather  tightly  over  the  primary  wire,  and  the 
last  edge  stitched.  The  whole  core  and  primary 
winding  should  then  be  soaked  in  hot  paraffin 
wax  until  the  absence  of  bubbles  shows  that  it 
has  penetrated  all  parts. 

Winding.  Two  ways  of  winding  the  secondary 
wire  are  allowable.  The  best  is  undoubtedly  that 
shown  and  described  in  the  next  chapter  for  a 
much  more  powerful  coil,  but  this  is  perhaps  un- 
necessarily troublesome  for  coils  up  to  1 in.  spark 


HOW  TO  MAKE  A ^-IN.  SPARKING  COIL.  27 

length.  The  other  consists  in  winding  in  four 
sections  only,  which  implies  a somewhat  different 
treatment.  The  method  of  winding  ‘ in  layers  ’ 
from  end  to  end,  as  described  in  the  previous 
chapter,  is  much  to  be  deprecated,  as  it  is  too  liable 
to  lead  to  breakdowns — particularly  annoying  if  the 
coil  is  used  for  ignition  of  the  charge  in  a motor 
car  engine. 

It  may  here  be  remarked  that  a smaller  coil — 
for  l in.  spark — might  be  built  in  two  sections 
only;  but  up  to  1 in.,  four  sections  should  be 
employed. 

For  the  four  sections  of  the  present  coil,  make 
four  spools  each  barely  1 in.  long  and  2f  in.  dia- 
meter. These  are  made  by  winding  three  or  four 
layers  of  paper  on  top  of  the  finished  primary, 
securing  with  glue  or  shellac  and  glueing  on  cir- 
cular cardboard  ends;  the  bobbin  (shown  in  Fig.  17) 
should  be  removed  from  the  primary  to  dry,  and 
is  then  to  be  well  soaked  in  paraffin  wax. 

A wooden  mandrel  should  be  turned  on  which 
the  spools  will  just  fit,  and  this  should  either  be 
mounted  in  the  lathe  or  in  a simple  winding 
machine.  On  the  whole,  it  will  be  found  advis- 
able not  to  soak  the  spool  of  wire  in  wax  before 
winding,  as  when  that  is  done  it  cannot  then  be 
wound  so  closely.  About  f lb.  of  No.  36  D.C.C. 
wire  will  be  needed;  silk-covered  wire  would,  of 
course,  give  better  results,  but  its  cost  hardly  war- 
rants its  use. 


28 


INDUCTION  COILS. 


A tiny  hole  should  be  made  in  one  of  the  ends 
of  each  spool  close  to  the  central  paper  cylinder; 
another  similar  hole  is  to  be  made  in  the  other 
ends  near  the  outer  edge.  These  holes  are  best 
made  with  a red-hot  needle,  and  the  burr  (if  any) 
should  be  removed  inside  and  out.  The  winding 
should  be  as  close  and  even  as  possible,  and  if  the 
first  layer  is  well  done  the  second  may  be  continued 
back  over  it.  It  will  then  almost  certainly  be 
necessary  to  cover  the  wire  with  a layer  of  thinnish 
paper  (one  turn  is  all  that  is  needed),  as  otherwise 
any  gap  between  adjacent  turns  will  allow  a turn 
of  wire  to  sink  down  into  contact  with  those  in  the 
first  layer,  the  difference  of  potential  being  then 
sufficient  to  break  down  the  insulation  to  that  part. 
The  paper  may  be  secured  by  a little  shellac,  and 
the  third  layer  of  wire  wound  over  it. 

Quite  possibly  it  will  be  found  that  the  winding 
will  ‘ pull  ’ the  paper  by  causing  it  to  sink  between 
the  turns  of  the  previous  layer.  This  will  result  in 
leaving  the  end  of  that  layer  bare  (see  Fig.  18). 
It  is  important  to  cover  this  with  a narrow  strip  of 
paper  or  thin  tape  before  winding  over  it.  Paper 
should  cover  every  layer  that  is  not  quite  perfect, 
before  proceeding  to  wind  the  next  layer. 

Spools.  All  four  spools  are  to  be  wound  in  ex- 
actly the  same  direction,  but  they  may  advantage- 
ously contain  different  quantities  of  wire.  The 
outer  spools  might  each  contain  about  two-thirds 
the  amounts  on  the  others  as  indicated  by  dotted 


HOW  TO  MAKE  A ^-IN.  SPARKING  COIL.  29 

lines  in  Fig.  19.  When  all  are  finished  the  spools 
must  be  well  soaked  in  hot  paraffin  wax;  if  carefully 
done  the  wax  will  penetrate  every  part  of  the 
winding  and  form  a solid  mass  of  the  whole.  The 
four  spools  are  now  to  be  assembled  on  the 
finished  primary.  Their  arrangement  will  be 
shown  in  Fig.  19  the  two  lighter  spools  on  the  ends 
of  the  coil.  They  must  be  put  on  in  a definite 
order  as  shown,  the  end  spools  having  their  finish- 
ing ends  outside  and  the  finishing  ends  of  the 


Fig.  19. 


middle  coils  to  be  in  the  middle  space.  Just  a 
sufficient  length  of  wire  should  be  led  outside  the 
spools,  cleaned,  scraped,  twisted  together  and 
soldered.  These  joints  are  to  be  pushed  down 
between  the  spools,  which  can  then  all  be  closed 
up  on  middle  of  the  core. 

Testing  the  spools.  Tests  have  been  presumably 
been  made  of  the  whole  secondary  spool  of  wire 
and  of  the  separate  coils  before  assembling  them. 
They  may  now  be  again  tested  by  joining  the  elec- 
trodes of  a battery  to  the  primary  winding,  the  sec- 
ondary ends  being  held  about  \ in.  apart. 


30 


INDUCTION  COILS. 


On  breaking  the  primary  circuit,  a spark  should 
jump  the  air  gap  at  the  secondary  electrodes, 
but  will  not  much  exceed  the  length  named  until 
the  condenser  has  been  fitted. 

The  condenser  consists  of  about  40  sheets  of 
tin-foil  7 in.  X 3 in.,  interleaved  with  waxed 
papers  8J  in.  X 4£  in.,  constructed  and  connected 
as  elsewhere  described.  With  a suitable  battery 
(two  good-sized  bichromate  cells  or  two  accumu- 
lators such  as  are  used  for  motor  cycle  work)  a 
i in.  spark  should  be  easily  obtained,  but  with 
four  dry  cells  of  the  usual  motor  cycle  type  the 
length  would  not  be  so  great  unless  the  battery 
is  quite  new. 

To  finish  the  coil,  fit  wooden  ends  over  the  core 
close  against  the  primary  winding.  These  ends 
may  be  3 in.  diameter  or  square,  and  f in.  or 
\ in.  thick.  If  the  coil  is  for  experimental  use,  it 
may  be  fixed  to  a baseboard  by  screwing  through 
to  the  wooden  ends.  The  two  primary  ends  are 
brought  out  and  taken  down  through  the  base- 
board, and  the  secondary  wires  can  be  carried 
to  terminals,  as  in  the  case  of  the  medical  coil 
previously  described.  A contact  breaker  of  the 
usual  type  should  be  fitted,  and  the  condenser 
placed  in  a shallow  b6x  below  the  baseboard.  The 
coil  can  be  finished  off  by  a turn  of  paper-ebonite 
cemented  in  place. 

For  motor  car  work  the  coil  may  be  fitted  with 
ends  about  3^  in.  diameter.  The  secondarv  should 


HOW  TO  MAKE  A J-IN.  SPARKING  COIL.  31 

be  covered  with  paraffin  wax  until  it  presents  a 
smooth,  cylindrical  surface  2§  in.  in  diameter. 
Round  this  and  between  the  ends,  the  condenser 
may  very  well  be  fixed,  first  warming  it  through 
to  make  it  pliable,  and  this  will  constitute  a neat 
and  compact  coil  for  the  work.  Several  sheets  of 
paraffined  paper  may  surround  the  whole,  and  a 
final  immersion  in  hot  wax  will  make  the  coil  quite 
impervious  to  water. 


CHAPTER  V. 

The  Construction  of  a 4-in.  Sparking  Coil. 

The  materials  and  tools  required  are  simple  and 
few  in  number.  The  ordinary  light  wood  and 
metal  working  tools  possessed  by  the  average 


amateur  should  suffice  to  turn  out  a well  finished 
and  efficient  coil.  The  aid  of  a lathe  will  greatly 
facilitate  the  construction,  but  it  is  by  no  means 
indispensable. 

Materials  required.  The  first  thing  to  do  will 
be  to  obtain  all  the  materials.  The  most  expensive 

32 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  33 

item  will  be  the  secondary  wire.  Obtain  4f  lbs. 
No.  36  D.C.C.  wire;  this  is  about  $2.25  per  lb. 
(silk  insulation  is  unnecessary).  For  the  primary 
1 J lb  No.  12  D.C.C.  wire  at  about  23  cents  per  lb., 
1J  lb.  of  thin  tin-foil  at  about  30  cents  per  lb., 
one  piece  of  hard  rubber  tube  8f  in.  by  If  | in. 
outside  diameter  and  J in.  thick,  $1.50.  Two 
pieces  of  sheet  hard  rubber  \ in.  thick  4f  in.  X 
4f  in.  at  about  $2.10.  1 lb.  No.  22  soft  iron  wire 

15  cents.  If  quire  white  filter  paper  (obtain  at 
chemical  warehouse)  30  cents;  a piece  of  No.  14 
platinum  wire  f in.  long  at  $1.30;  two  large  bind- 
ing posts;  two  small  ditto  at  about  10  cents  each; 
a few  square  inches  of  -fj  in.  sheet  brass  25  cents; 
4 in.  of  f in.  width  clock  spring;  one  contact  screw 
about  one  in.  long,  T3¥  thick  with  lock  nut;  one 
piece  of  soft  iron  rod,  § in.  in  diameter  and  1 in. 
long;  four  yfr  in.  machine  screws,  If  in.  long;  two 
| in.  machine  screws,  fin.  long;  two  Tsyin.  screws 
(brass),  § in.  long;  two  dozen  assorted  wood  screws, 
§ in.  to  fin.  long,  about  50  cents  the  lot;  a few 
square  feet  of  f in.  basswood  or  good  pine  for  the 
base,  25  cents;  2 lbs.  of  paraffin  wax,  about  15  cents 
per  lb.;  solder,  resin,  and  shellac  varnish.  These 
quantities  are  a liberal  allowance  for  a coil  this 
size,  and  are  intended  only  as  a guide  for  the 
amateur. 

Fig.  20  shows  a plan  of  base  for  this  coil. 

The  baseboard  of  the  apparatus  (19,  Fig.  21)  is 
in  the  form  of  a shallow  box,  12f  in.  by  7f  in.  by 


34 


INDUCTION  COILS. 


Fig.  21. 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  35 


2f  in.  deep.  It  will  be  a fairly  simple  piece  of 
work;  the  joints  being  either  screwed  or  dove- 
tailed together,  according  to  the  skill  of  the  worker. 
The  main  thing  is  to  make  it  strong  and' square. 
Smooth  up  the  sides  and  top  well,  so  that  it  can 


be  varnished.  It  will  be  seen  from  the  sketch . 
that  the  under  side  is  covered  in  by  a thin,  well- 
fitting board.  This  is  fixed  by  screws  to  fillets 
glued  into  the  corners  of  the  box. 

The  coil  ends  are  indicated  by  Nos.  4 and  10  in 
Fig.  21.  The  best  possible  material  for  these  is 


36 


INDUCTION  COILS. 


ebonite,  although  paraffined  oak  would  make  a 
good  substitute.  These  must  be  nicely  filed  up 
to  size,  and  the  edges  and  surfaces  finished  off 
with  fine  emery  cloth  and  oil.  Then  bore  a 13/16 
in.  hole  (a  good  fit  for  ebonite  tube)  a little  above 
the  centre  as  sketched.  This  can  be  done  in  the 
lathe  or  by  means  of  a fret-saw,  finishing  off  true 
with  a half  round  file.  Two  holes  must  now  be 
drilled  in  the  lower  edges  and  tapped  for  a v\- 
in.  screw;  also  one  hole  in  the  top  edge  for  the 
terminals  of  secondary  wire. 

Insulating  tube.  The  important  insulating  tube 
between  primary  and  secondary  (3,  Fig.  21) 
must  be  obtained  cut  to  size,  straight  and  circular; 
be  very  careful  that  no  minute  holes  perforate 
the  walls.  This  would  cause  a speedy  breakdown 
of  the  coil.  If  an  ebonite  tube  cannot'be  obtained 
make  one  by  wrapping  thin  ‘ paper  ’ ebonite 
around  a mandrel,  cementing  each  layer  with 
shellac  varnish. 

Core  wire.  For  the  soft  iron  wire  core  cut  the 
No.  22  iron  wire  into  8 lengths;  straighten  and 
make  into  a neat  round  bundle  1 in.  in  diameter. 
Next  pour  some  shellac  varnish  down  between 
the  wire,  and  dry  in  the  oven.  The  core  is  shown 
by  1,  Fig.  21.  Two  wood  flanges  must  now  be 
made  (No.  11)  so  as  to  fit  the  core  tight  at  the  ends 
and  slip  easily  into  the  ebonite  tubes.  Fix  on 
tight  so  as  to  leave  a space  of  7 in.  between  them. 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  37 

A layer  of  paper  should  now  be  cemented  around 
the  core  and  three  layers  of  No.  12  D.C.C.  wire 
wound  in  the  space.  Two  small  holes  will  require 
to  be  drilled  one  at  each  end,  12  in.  of  the  wire 
being  passed  through  the  hole  close  to  the  core 
from  the  inside ; the  end  of  the  third  layer  is  brought 
out  through  the  hole  drilled  in  the  edge  of  the 
opposite  flange.  Keep  the  winding  as  close  and 
tight  as  possible,  and  finally  give  it  a coat  of  shellac 
varnish. 

Fig.  22  shows  a cross-section  of  a 4-in.  spark  coil. 

A section  winder  will  be  required  for  making 
the  secondary  sections,  No.  8a  in  the  figure.  This 
will  be  understood  by  looking  at  the  sketch,  Fig. 
23.  It  consists  of  two  discs  of  hard  wood,  3f  in. 
diameter,  separated  not  more  than  J in.  by  a disc 
of  metal  l{-f  in.  diameter.  The  three  are 
fixed  upon  a screw  spindle  and  clamped  together 
by  nuts.  This  has  now  to  be  mounted  in  uprights 
secured  to  a wood  base.  A small  handle  is  bent 
or  attached  to  the  end  of  the  spindle.  This  being 
made,  the  spools  of  No.  36  wire  must  be  well 
saturated  with  hot  paraffin.  This  will  best  be  done 
by  obtaining  a metal  vessel  deep  enough  to  hold 
the  spools.  Melt  the  paraffin  carefully  in  it  and 
then  immerse  the  spools  in  it  till  no  more  air- 
bubbles  are  driven  out;  the  spools  may  then  be 
hung  up  to  drain.  Now  fix  the  winder  securely 
to  the  table,  and  fix  a stout  wire  horizontally  and 
^bout  2 ft.  above  the  table.  On  this  the  spool 


38 


INDUCTION  COILS. 


is  placed,  and  a Bunsen  burner  fixed  underneath; 
the  hot  air  rising  from  it  will  render  the  paraffin 
on  the  wire  soft.  Secure  a turn  of  wire  around 
the  centre  disc  of  the  winder,  and  proceed  to  care- 
fully wind  until  the  space  is  full.  Then  cut  the 
wire  and  remove  the  inner  disc  of  the  winder;  the 
wire  section  will  readily  come  away  from  the 


Fig.  23. 


disc,  this  being  tapered  for  the  purpose.  The 
wax  will  reset  on  the  turns  of  wire  and  hold  them 
quite  firm.  Forty  sections  must  be  made,  and  it 
would  be  advisable  to  test  each  one  for  continuity 
with  battery  and  galvanometer  or  a magneto  before 
mounting. 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  39 


The  discs  insulating  one  secondary  section  from 
the  next  (No.  8 in  Fig.  21)  are  to  be  made  from 
filter  paper,  and  soaked  in  paraffin  wax.  For 
melting  the  wax,  obtain  a shallow  baking  tin, 
which  should  not  be  less  than  11  in.  X 7 in.  As 


the  condenser  sheets  will  also  require  paraffining 
make  a true  cardboard  gauge,  4 in.  diam.  by 
l^f  in.  hole  in  centre ; place  this  over  a number 
of  the  sheets  together,  and  cut  through  with  a 
sharp  penkni-fe.  About  ninety  discs  will  be  re- 


40 


INDUCTION  COILS. 


quired.  Next,  have  the  paraffin  nicely  melted, 
and  soak  the  discs  in  it;  take  them  out  one  at  a 
time,  allow  to  set  for  an  instant,  and  then  place 
them  on  a clean  sheet  of  paper  to  cool.  It  is 
important  that  no  dust  or  metallic  particles  ad- 
here to  the  surfaces. 


Fig.  25. 


The  condenser  (Fig.  25)  consists  of  sixty  sheets 
of  tin-foil  interleaved  with  paraffined  paper.  Cut 
the  paper  10  in.  X 6^  in.,  and  paraffin  them  as 
before.  The  foil  sheets  must  be  cut  to  size  (8  in. 
X 5 in.)  with  a connecting  lug  as  sketched.  This 
may  be  done  by  first  cutting  the  sheet  \ in. 
longer  and  slitting  it  across  to  within  1 in.  of  the 
edge,  then  simply  bend  back  the  lug  thus  formed. 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  41 


Proceed  to  build  up  the  condenser  by  first  cutting 
two  pieces  of  cardboard,  10  in.  X 6^  in.  Lay 
one  flat  on  a level  table;  on  the  top  place  a sheet 
of  paraffined  paper;  next  place  a foil  sheet  sym- 
metrical with  it,  with  its  lug  projecting  over  the 
edge;  over  this  a paper  sheet,  next  a foil  sheet, 
with  its  lug  at  the  opposite  lower  corner,  and  so 
on  alternately,  till  the  full  number  are  built  up. 
Fasten  the  lugs  on  each  side  firmly  together. 
The  other  cardboard  sheet  is  now  to  be  placed  on  top. 
Heavily  weight  it  for  some  hours,  and  after- 
wards tie  firmly  together  with  tape. 

Contact  breaker.  The  construction  of  the  con- 
tact breaker  or  interruptor  (21  and  24)  will  be 
readily  understood  from  the  sketch,  Fig.  24.  The 
soft  iron  armature  is  made  from  a piece  of  § in. 
round  iron  1 in.  long.  This  is  drilled  and  tapped 
and  secured  to  the  spring,  this  being  rigidly 
attached  to  a brass  angle  plate.  A screw  passes 
through  so  as  to  press  against  the  spring  and 
increase  the  tension  if  necessary.  The  head  of 
the  armature  screw  must  be  filed  down,  and  a 
small  hole  drilled  in  its  centre  to  allow  a small 
piece  of  No.  14  platinum  wire  being  driven  in 
tight.  The  contact  screw  end  must  be  tipped 
in  the  same  manner.  This  screw  is  supported  by 
a long  angle-piece,  into  which  it  screws  nicely. 
A lock  nut  should  be  added  to  the  screw,  or  else 
a slot  cut  through  into  the  hole,  so  that  the  sides 
can  be  closed  up  on  to  the  screw  to  keep  it  firm. 


42 


INDUCTION  COILS. 


Holes  for  small  wood  screws  are  drilled,  as  shown, 
into  the  bases  of  the  plates;  also  a third  hole,  to 
allow  of  a connecting  pin  being  screwed  or  soldered 
in. 


The  commutator  or  reverser  is  not  absolutely 
necessary,  but  is  a useful  addition  to  the  coil. 
The  form  illustrated  in  Fig.  26  is  as  good  as  any. 
It  consists  of  a short  cylinder  of  ebonite  or  hard 
wood  E,  through  the  centre  of  which  passes  a 
brass  pin  F.  This  is  really  in  two  parts,  so  as 
to  be  insulated  from  each  other.  Two  contact 
plates  are  screwed  to  opposite  sides  of  the  cylin- 
ders, and  one  put  into  contact  with  each  of 
the  pins  by  a screw  passing  through  it  The 
cylinder  is  supported  by  two  angle-pieces  A and 
B,  and  two  brass  springs  C and  D,  arranged  to 
press  against  the  contact  plates.  An  ebonite 
or  brass  handle  is  attached  to  the  spindle.  Plates 
A and  B form  the  terminals,  and  the  springs  C 
and  D are  attached — one  to  the  free  end  of  the 
primary  coil  and  the  other  to  the  contact  screw. 
It  will  readily  be  seen  that  the  springs  C and  D 
can  be  put  into  contact  with  either  poles  of  the 
battery  at  will  by  simply  turning  the  cylinder 
round. 

Building  up  secondary.  The  most  important 
detail  in  fitting  the  parts  together  is  to  build  up 
the  secondary.  For  this  a small  vessel  of  melted 
paraffin,  soldering  iron,  solder,  resin,  and  a warm 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  43 


laundry  iron  will  be  required.  Have  all  the  discs 
and  sections  at  hand,  and  fix  the  ebonite  tube 
into  one  of  its  flanges,  so  as  to  pass  through  J in., 
and  stand  it  up  vertical.  Slip  three  or  four  paper 
discs  over  the  tube  and  flat  against  the  flange; 
then  fix  a section  in  position,  withdraw  inner  end 


of  the  wire  and  arrange  it  concentric  with  the  tube. 
The  space  (3a,  Fig.  21)  must  be  filled  up  solid 
with  melted  paraffin.  When  set,  place  two  in- 
sulating discs  on  top  (the  wire  being  brought 
up  through  them);  then  smooth  them  down  with 
the  warm  iron.  The  sections  must  be  connected — 
two  insides  together,  then  two  outsides,  and  then 
insides,  and  so  on.  Be  most  careful  to  get  the 


44 


INDUCTION  COILS. 


proper  face  down,  otherwise  some  of  the  sections 
will  be  opposing  each  other.  The  proper  way  is 
shown  on  diagram,  Fig.  27.  The  joints  will  re- 
quire to  be  soldered,  the  inner  joint  being  neatly 
tucked  in  the  space  between  section  and  tube. 
The  outer  joint  (No.  7)  is  brought  over  the  top  of 


discs,  and  then  slipped  in  between  them.  When 
all  the  sections  are  in  place,  bring  the  inner  end  of 
the  last  one  up  between  three  or  four  insulating 
discs,  and  fit  on  the  other  coil  flange.  Any 
space  between  it  and  the  last  sections,  when  the 
tube  is  projecting  through  equally  on  each  side, 
should  be  filled  up  with  more  paraffined  discs.  Two 
small  holes  are  now  to  be  drilled  slantwise  up 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  45 


through  the  flange  to  the  terminals  5 and  6,  and 
the  secondary  wires  passed  through  and  joined  ’ 
to  them. 


Assembling.  The  position  of  the  fastening 
down  holes  on  the  base  is  now  to  be  found,  and 
the  holes  drilled  to  allow  the  screws  to  pass  through 
into  the  holes  into  the  flanges  (17  and  18).  The 
primary  is  now  slipped  into  the  tube,  and  two 
tight-fitting  end  pieces  of  wood  (12)  made  to  fit 
the  tube.  One  must  have  a f in.  hole  drilled 
through  its  centre  (12),  and  both  will  require 
small  holes  drilling  to  pass  the  primary  wires 
through.  These  are  then  taken  through  holes  in 
the  baseboard.  The  contact  breaker  should  now 
be  fixed  in  position  as  shown  on  the  diagram, 
and  the  end  of  the  wire  nearest  soldered  to  the 
brass  pin  in  the  armature  support.  The  contact 
screw  is  taken  direct  to  one  of  the  large  terminals 
(16),  the  other  terminal  making  contact  to  the 
free  end  of  the  primary  (4a).  The  lugs  of  the 
condenser  are  put  in  contact,  one  to  each  contact 
pillar  and  spring.  Make  a good  connection  to 
foil  lugs  by  wrapping  some  No.  24  tinned  copper 
wire  tightly  around  them  and  making  a soldered 
connection  to  each  of  the  pins  (22  and  23).  The 
condenser  can  be  wedged  in  place  with  a few  strips 
of  wood  and  the  wood  base  cover  screwed  on 
when  all  is  secure  inside. 


46 


INDUCTION  COILS. 


The  covering  for  the  secondary  consists  best  of 
‘ paper  ’ ebonite,  cut  to  a good  fit  between  the 
flanges  and  made  to  overlap  about  1 in.  The  lap 
is  cemented  with  strong  shellac  varnish,  a few 
turns  of  string  keeping  it  in  place  till  set.  An  ex- 
cellent substitute  for  ebonite  is  paraffined  cartridge 
paper  fixed  in  the  same  way,  afterwards  varnished 
black.  The  coil  can  now  be  tested.  Arrange  the 
armature  to  be  about  in.  from  the  core,  and 
adjust  the  contact  screw,  so  that  when  the  armature 
touches  the  core  the  circuit  is  broken  in. 
Next  fix  a short  piece  of  wire  in  each  secondary 
terminal,  so  that  they  stand  4 in.  apart.  On  con- 
necting three  large  bichromate  cells  in  series  on  to 
the  primary  terminals,  the  interruptor  should 
vibrate  and  produce  a torrent  of  sparks  between 
secondary  terminals.  If  the  full  length  of  spark 
is  not  obtained  right  off,  try  putting  more  or  less 
tension  on  the  armature  spring  till  successful. 
No  difficulty  should  occur  in  obtaining  a full  4-J-  in. 
spark  if  the  directions  are  closely  followed.  If 
storage  batteries  are  used  to  work  the  coil,  place 
a small  resistance  in  series,  otherwise  the  platinum 
contacts  will  burn  away  quickly,  due  to  the  heavy 
current  on  short  circuiting  at  the  contacts;  also 
be -careful  not  to  overwork  the  coil  with  too  many 
cells. 

The  appearance  of  the  coil  will  be  greatly 
improved  by  finishing  off  the  brass  work,  such  as 
the  contact  breaker,  commutator  and  terminals, 
to  as  high  a polish  as  possible;  then  lacquer  the 


CONSTRUCTION  OF  A 4-IN.  SPARKING  COIL.  47 


parts  with  a good  white  or  gold  metal  lacquer. 
The  commutator,  if  used,  is  best  placed  in  the 
position  shown  in  the  baseboard  plan. 

A word  of  warning  should  also  be  given  against 
using  the  coil  on  high  voltage  circuits  with  the 
electrolytic  interruptor.  The  insulation  is  almost 
certain  to  be  broken  down  if  the  current  is  kept  on 
more  than  a few  seconds.  This  is  due  to  the  in- 
tense pressure  induced  in  the  secondary,  and  as 
the  current  is  also  fairly  heavy  for  the  wire  to 
carry,  it  is  liable  to  warm  it  up  and  soften  the 
paraffin,  and  thereby  allow  a spark  to  perforate  it. 

This  coil  will  stand  the  use  of  an  electrolytic 
interruptor  very  well  if  the  following  changes 
in  the  construction  of  the  apparatus  are  made. 
Instead  of  D.C.C.  wire  on  the  secondary,  use 
D.S.C.  wire.  In  place  of  the  paraffined  paper 
insulation  between  each  section  of  the  secondary, 
mica-disks  in.  thick,  or  “ micanite  ” in. 
thick,  should  be  used.  These  changes  will  bring 
the  cost  of  the  induction  coil  about  $12  to  $13 
higher. 


CHAPTER  VI. 


How  to  Increase  the  Efficiency  of  Sparking 
Coils. 

The  principal  fault  to  be  found  in  coils  of  good 
design  and  construction  is  in  regard  to  the  con- 
tact breaker.  The  ordinary  form  so  commonly 
fitted  because  of  its  great  simplicity  is  to  be  seen 
in  the  drawing  of  the  various  coils  described  in 
this  book.  The  trouble  with  this  type  of  break 
is  that  its  armature  is  usually  attracted  by  the 
soft  iron  core  of  the  coil  before  this  latter  has 
really  had  time  to  become  thoroughly  magnetized, 
and  as  on  this  degree  of  saturation  the  effect  of 
induction  largely  depends,  it  follows  that  the  full 
power  of  the  coil  is  not  called  into  play.  Various 
means  have  been  adopted  for  the  purpose  of  pro- 
longing the  contact  to  enable  the  core  to  feel 
the  full  effect  of  the  current  in  the  primary  wind- 
ing; and  sometimes  the  substitution  of  a strong 
for  a weak  spring  will  effect  a very  considerable 
improvement  in  the  sparking  length  of  a given  coil. 

Sparking  troubles.  Another  trouble  with  large 
coils  especially  is  the  destructive  sparking  when 
the  ordinary  break  is  employed.  This  results 
in  a comparatively  uncertain  contact  between  the 
platinum  surfaces,  which  becomes  very  marked 

48 


EFFICIENCY  OF  SPARKING  COILS. 


49 


after  the  coil  has  been  used  for  some  time,  the 
platinum  becoming  volatilized,  and  depositing 
on  the  negative  side  of  the  contact,  making  the 
surfaces  very  uneven,  and  having  a burnt  appear- 
ance. Consequently,  unless  they  are  trimmed 
up  with  a smooth  file  occasionally,  the  full  cur- 
rent necessary  to  magnetize  the  coil  cannot  pass 
the  contacts. 

Mercury  interruptor.  These  defects  are  absent 
in  the  type  of  break  called  a mercury  interruptor, 
which  can  be  readily  adapted  to  any  coil  without 
interfering  with  the  construction  in  the  least. 
The  principal  of  the  mercury  break  is  that  de- 
vised by  Foucault  many  years  ago,  and  which 
has  recently  been  revived  in  several  modified  and 
unnecessarily  complicated  forms.  In  the  type 
here  described  and  illustrated  in  Fig.  28,  the  lines 
of  the  original  break  are  followed,  but  the  vibrat- 
ing wire  or  contact  rod  is  actuated  by  an  independ- 
ent electro-magnet,  instead  of  utilizing  the  core. 
The  parts  and  materials  for  constructing  this 
break  are  of  a very  simple  character,  and  they 
are  easily  fitted  together  in  the  following  manner: 

Obtain  an  ordinary  bell  magnet  with  armature 
and  contact  complete,  but  substitute  good  massive 
platinum  contacts  if  these  are  too  light.  The 
magnet  should  be  wound  with  plenty  of  fairly 
thin  wire.  Make  a suitable  baseboard,  say  7 in. 
X 4 in.  X i in.,  and  get  also  about  6 in.  of  § in. 
brass  rod,  four  terminals,  two  4-oz.  wide-mouthed 


50 


INDUCTION  COILS. 


glass  bottles,  two  feet  of  No.  10  bare  copper  wire, 
a few  inches  of  glass  rod,  and  some  mercury. 
The  diagram,  Fig.  28,  will  indicate  the  method  of 
fitting  up.  The  magnet  is  to  be  mounted  by 
means  of  a tapped  hole  in  the  yoke  on  top  of  the 


brass  rod,  the  end  of  which  is  suitably  screwed. 
The  other  end  of  this  brass  rod,  R,  is  screwed 
firmly  into  the  baseboard.  The  two  phials  are 
fixed  close  together  in  recesses  cut  in  the  base, 
and  a portion  of  the  No.  10  copper  wire  is  bent  to 


EFFICIENCY  OF  SPARKING  COILS. 


51 


a U-shape,  so  that  one  leg  dips  into  each  bottle, 
the  top  of  this  U being  securely  soldered  to  the 
hammer  shaft  of  the  magnet.  The  leg,  C,  dips 
nearly  to  the  bottom  of  its  jar;  B is  pointed  and 
goes  to  within  half  an  inch  of  the  bottom.  A 
cork  being  fitted  in  the  jar,  P,  has  in  it  three 
holes,  one  large  enough  to  admit  B freely  without 
any  chance  of  binding;  and  in  the  second  is  fitted 
the  glass  rod  G,  which  can  slide  up  and  down. 
A permanent  connection  of  No.  10  or  12  copper 
wire  must  reach  to  the  bottom  of  the  jar  D,  and 
be  connected  to  the  terminal  A;  a similar  arrange- 
ment being  made  also  in  regard  to  the  second  jar 
P,  J in.  of  clean  mercury  is  then  placed  in  each 
phial,  and  also  1 in.  deep  of  alcohol  (methylated 
spirits)  in  P.  The  purpose  which  the  alcohol 
serves  is  to  extinguish  the  electric  arc  or  spark 
which  is  formed  while  breaking  the  current. 

The  magnet  terminals  are  joined  up  to  a 
couple  of  Leclanche  cells,  and  E,  A connected, 
one  each,  to  the  existing  contact  and  spring 
pillars.  The  platinums  must  be  separated  by  a 
small  bit  of  cardboard,  and  the  screw  turned 
till  there  can  be  no  movement  of  the  armature. 
The  usual  battery  is  then  joined  up  to  the  coil 
terminals,  and  a few  trial  adjustments  of  the 
speed  of  interruption  by  the  armature  contact 
and  the  wire  dipping  into  the  mercury  made, 
when  excellent  results  should  be  obtained.  The 
object  of  the  glass  rod  G is  to  enable  a greater  or 


52 


INDUCTION  COILS. 


less  height  of  mercury  to  be  obtained  by  pushing 
the  rod  down  or  up  as  required. 

Dimensions.  The  exact  dimensions  of  the  vari- 
ous parts  are  not  of  great  importance,  and  the 
method  of  fitting  up  should  easily  be  understood 


from  the  diagram.  This  is  approximately  one- 
third  size,  and  is  suitable  for  coils  between  2-in. 
and  12-in.  spark. 

Wehnelt  interruptor.  Better  than  any  mechan- 
ical circuit  breaker  for  very  large  coils  is  the  very 
modern  apparatus  known  as  the  Wehnelt  inter- 


EFFICIENCY  OF  SPARKING  COILS. 


53 


ruptor.  A practical  form  of  this  is  illustrated  in 
Figs.  29  to  33,  and  may  be  made  as  follows: 

Construction.  To  construct  the  interruptor  as 
shown  in  Fig.  30,  procure  a square  glass  (or 
earthenware  (accumulator  cell  measuring  about 
5 in.X3  in.X3  in.,  and  from  mahogany,  § in.  in 
thickness,  cut  a piece  to  form  for  it  a lid  or  cover 
4 in.  square  to  allow  \ in.  overhang  all  round. 


On  a centre  line  drawn  across  its  grain,  cut  an 
aperture  1£  in.  longXi  in.  wide,  and  at  one  end, 
and  right  angles  thereto,  cut  another  1£  in.Xi  in., 
thus  making  the  complete  aperture  T-shaped. 

The  cover.  On  that  side  of  the  cover  which 
ultimately  will  be  innermost,  screw  on  two  narrow 
fillets,  one  at  each  side  of  the  larger  part  of  the 
opening,  in  such  a manner  that  their  ends  and  one 
side  will  just  butt  against  the  upper  edges  of  the 


54 


INDUCTION  COILS. 


vessel,  so  as  to  serve  the  twofold  service  of  forming 
a step  for  the  cover  and  a prevention  of  warping. 
Fig.  30  shows  this,  the  dotted  line  indicating  the 
top  of  the  vessel.  The  under  side  of  the  cover 
should  be  rendered  impervious  to  steam  and 
water  by  enamelling  it  two  or  three  coats,  and 
the  edges  and  upper  side  finished  by  polishing  or 
varnishing. 

Next,  cut  a piece  of  close-grained  cork  to  the 
shape  shown  in  Fig.  31.  Virtually  it  is  a 1-in. 
cube  rabbetted  \ in.  deep  on  either  side  to  fit 
and  slide  rather  stiffly  in  the  larger  slot  in  the 
cover. 

The  tube.  Take  a piece  of  uranium  glass  tubing 
of  \ in.  diameter  by  8 in.  or  9 in.  long,  and  with 
the  aid  of  a spirit-lamp  or  gas  Bunsen  flame  heat 
it  near  one  end  until  soft.  Before  it  has  time  to 
cool,  stop  the  end  with  the  finger-tip,  and  blowing 
gently  with  the  lips  at  the  other,  bend  it  to  as 
near  a right  angle  as  possible.  The  object  of 
blowing,  it  may  be  mentioned,  is  to  prevent  the 
walls  of  the  tube  in  their  plastic  condition  from 
collapsing;  on  the  other  hand,  if  blown  too  vio- 
lently, an  unsightly  bulb  probably  will  appear. 

The  tube,  at  this  stage,  should  be  as  at  A, 
Fig.  32.  At  a distance  of  about  § in.  from  the 
bend,  re -heat  the  tube  and  carefully  draw  in  it  a 
narrow  contraction  by  pulling,  ensuring,  in  so 
doing,  to  keep  the  part  perfectly  straight  (see  B, 
Fig.  32).  When  cool,  take  the  tube  in  both  hands. 


EFFICIENCY  OF  SPARKING  COILS. 


55 


and  with  thumb  nails  together  at  the  point  marked 
by  a dotted  line  in  the  figure,  break  it  cleanly  in 
two.  Insert  in  the  small  orifice,  which  the  tube 
will  then  have,  a piece  of  No.  20  B.  & S.  platinum 
wire  about  § in.  long,  projecting  \ in.  or  f in., 
and  secure  it  by  playing  on  the  tip  of  the  glass 
with  a very  fine  blowpipe  flame.  Let  the  glass 
accumulate  only  sufficiently  to  hold  the  wire  in 
place,  as,  should  it  become  much  thickened,  it  will 
be  liable  to  fracture  when  in  use.  The  instant  the 
final  operation  is  completed,  thickly  cover  the 
tube  from  point  to  bend  with  the  deposit  of  a 
smoky  gas  flame.  Beneath  this  carbonaceous  en- 
velope cooling  will  progress  comparatively  slowly, 
so  that,  for  all  practical  purposes,  the  glass  will 
be  thoroughly  annealed — a most  desirable  quality, 
it  is  to  be  noted. 

Bore  a hole  in  the  piece  of  cork  previously 
fitted  to  the  vessel’s  lid,  and  thrust  the  upper  end 
or  straight  part  through  it  from  its  under  side, 
adjusting  the  tube  in  the  hole,  which,  by  the  way, 
it  should  fit  tightly,  so  that  the  platinum  point 
may  reach  about  midway  down  the  depth  of  the 
bath.  A piece  of  sheet-lead  must  now  be  cut  to 
the  form  and  dimensions  given  in  Fig.  33,  a suit- 
able gauge  for  this  being  No.  14  B.  & S.,  or  about 
in.  thick,  though  it  is  scarcely  necessary  to 
be  particular  within  one  or  two  numbers  of  that 
guage. 

Cut  a small  wedge-shaped  piece  of  any  wood 
1{  in.  wide  1 in.  long,  and  tapering  from  \ in.  to 


56 


INDUCTION  COILS. 


J in.  Place  the  lug  of  the  lead  plate  through  the 
smaller  slot  of  the  lid  from  underneath,  fasten 
it  firmly  into  position  from  above  with  the  wedge, 
and  finish  that  electrode  by  clamping  to  its  up- 
standing portion  an  appropriate  brass  binding- 
screw. 


Fig.  32. 


Fig.  33. 


A terminal  for  the  anode  may  be  screwed  into 
the  cover  on  the  side  opposite  to  the  cathode. 
Twist  a piece  of  fairly  heavy-gauge  silk-covered 
copper  wire  into  spiral  form,  bare  both  ends, 
bending  one  to  the  shape  of  a ring  that  it  may  be 
clamped  securely  under  the  base  of  the  binding- 
screw,  and  straightening  the  other  to  dip  an  inch 


EFFICIENCY  OF  SPARKING  COILS. 


57 


or  oO  into  the  top  of  the  glass  tube.  To  complete 
the  interruptor  for  use,  fill  the  vessel  to  within  f 
in.  of  the  top  with  water  and  sulphuric  acid  in  the 
proportions  of  8 to  1,  and  pour  sufficient  pure 
mercury  into  the  tube  to  enable  connection  to  be 
made  between  the  platinum  point  and  the  dipping 
wire. 

Electrodes.  Generally  it  will  be  found  con- 
venient to  make  several  platinum  electrodes,  each 
with  its  distinctive  feature,  such  as  length  and 
thickness  of  the  point,  its  degree  of  sharpness, 
and  the  reverse.  If  one  only  is  made,  it  is  well  to 
decide  upon  the  e.m.f.  of  the  current  usually  to 
be  applied,  and  then  to  shorten  or,  it  may  be, 
lengthen  the  piece  of  platinum  until  the  best  effect 
on  that  particular  circuit  is  attained.  Needless 
to  say,  the  former  course  is  the  better  of  the  two. 

When  first  trying  the  break,  the  bath  being 
cold,  switch  on  a current  of  from  2 to  3 amperes 
at  a potential  difference  of  about  45  volts.  Fig. 
34  shows  another  very  efficient  form  of  electro- 
lytic interruptor,  which  is  easily  made,  and  should 
not  cost  more  than  75  cents  complete.  The  in- 
terruptor is  so  simple  that  it  needs  very  little  ex- 
planation. A is  a glass  or  porcelain  jar  5 or  6 in. 
high,  about  4 in.  in  diameter;  B a porcelain  cup 
or  beaker  4 in.  X2  in.  or  2\.  A glass  rod  C § in. 
thick  with  a point  on  one  end.  Into  each  jar 
a lead  electrode  D and  E are  placed  which  have 
the  shape  as  shown  in  F.  Two  covers  are  fitted 


58 


INDUCTION  COILS. 


to  the  jars  G H.  G has  a hole  to  receive  a cork 
I which  holds  the  glass  rod  in  place.  Openings 
must  be  made  at  L and  M into  these  covers  to  pass 
the  lead  terminals  through.  The  cup  B has  a 
hole  in  the  bottom  not  larger  than  -fa  in.  This 


Fig.  34. 


hole  K receives  the  point  of  the  glass  rod  C which 
acts  as  a valve.  After  the  apparatus  is  assembled 
it  is  filled  about  § full  with  diluted  sulphuric 
acid  8 to  1.  It  is  ready  now  for  connecting  up  and 
can  be  tried.  To  start  push  the  glass  rod  as  far 


EFFICIENCY  OF  SPARKING  COILS. 


59 


down  into  the  hole  K as  possible.  The  effect 
and  operation  of  this-  interruptor  is  the  same  as 
Fig.  29. 

It  must  be  remembered  that  in  using  an  electro- 
lytic interruptor  no  condenser  is  necessary. 

The  action  of  the  apparatus  is  peculiar;  the 
interruptions  of  the  current  being  caused  appa- 
rently by  an  incredibly  rapid  formation  and 
disruption  around  the  anode  of  a gaseous  sheath. 
The  circuit  into  which  it  is  inserted  should  possess 
a certain  degree  of  self-induction,  as  otherwise 
it  will  fail  to  operate.  It  is  also  upon  this  in- 
duction, the  electro-motive  force  of  the  current 
and  the  surface  area  of  the  anode,  that  the  fre- 
quency of  the  interruptions  or  oscillations  depend, 
it  being  possible,  by  the  relative  alteration  of 
these  three  factors,  to  effect  wide  variations. 

Examination  with  a rotating  mirror,  such  as 
is  usually  employed  for  determinations  of  this 
kind,  has  shown  that  the  interruptions  may  reach 
if  not  actually  exceed,  the  extraordinary  number 
of  1500  per  second.  This  admirably  fits  the  break 
for  radiographic  work,  the  fluorescent  screen  par- 
ticularly, since  at  such  a rapidity  there  can  be 
no  flickering  of  the  light  sensible  to  the  eye. 

It  has  been  found,  in  experimenting  with  the 
Wehnelt  contact  breaker,  that  the  potential  dif- 
ference of  the  circuit  must  be  greater  than  that 
which  customarily  suffices,  though  the  voltage  may 
need  but  slight  augmentation  if  the  electrode  bath 
be  heated  to  about  150°  F. — a quality  self-acquired 


60 


INDUCTION  COILS. 


to  some  extent,  after  a short  period  of  use;  that 
the  secondary  spark  or,  as  it  more  resembles, 
flame  discharge  of  the  coil  is  of  extreme  heat, 
whilst,  also,  its  thickness  far  surpasses  that  which 
could  be  obtained  by  using  a hammer,  or  indeed, 
any  mechanical  break. 

Owing  to  the  heat  of  the  discharge,  the  current 
must  be  regulated  with  the  greatest  nicety,  and 
for  this  purpose  it  is  advisable  to  insert  in  the 
circuit  either  a choking  coil  or  a rheostat  and 
suitable  measuring  instruments. 

From  two  to  three  or,  at  most,  four  amperes  of 
current  will,  in  the  majority  of  cases,  be  found 
sufficient  for  radiography,  a heavy  current  being 
most  destructive  to  the  vacuum  tubes. 

Whilst  a good  coil  can  be  much  increased  in 
usefulness  by  the  adoption  of  special  apparatus 
as  here  described,  it  should  always  be  remembered 
that  the  secret  of  building  a good  coil  with  the 
minimum  amount  of  secondary  wire  is  to  have  it 
most  thoroughly  insulated,  and,  at  the  same  time 
well  arranged  with  regard  to  its  primary  winding. 
In  other  words,  the  closer  the  secondary  can  be 
got  to  the  primary  winding,  always  supposing 
the  insulation  to  be  perfect,  the  better  will  be  the 
results  obtainable. 


CHAPTER  VII. 


Particulars  of  Coils  for  Various  Lengths  of 
Spark. 

The  following  tables  are  designed  to  assist 
readers  in  forming  an  estimate  of  the  dimensions 
they  should  choose  when  about  to  build  an  in- 
duction coil  to  give  a certain  length  of  spark. 
It  should  be  observed  that  these  particulars  are 
necessarily  of  an  approximate  character  only — 
the  ability  of  different  workers  being  extremely 
wide  in  range,  and  this  being  the  determining 
factor  in  induction  coil  construction.  Conse- 
quently, it  may  be  remarked  that  the  dimen- 
sions and  quantities  here  quoted  are  the  minimum 
that  should  be  employed  ordinarily,  and  they  will 
only  give  the  stated  results  under  conditions  of 
excellent  workmanship. 

The  most  suitable  primary  batteries  for  work- 
ing induction  coils  are  those  of  the  ordinary  single- 
fluid bichromate  type,  unless  a very  long  ‘ run  ’ 
is  anticipated,  when  the  double  fluid  or  ‘ constant- 
current  ’ bichromate  battery  is  better.  In  nearly 
all  cases  an  accumulator  is  the  most  satisfactory 
source  of  current,  but  is  not  always  available, 
and  the  only  primary  battery,  other  than  the  bi- 
chromate types  already  mentioned,  that  can  be 
regarded  as  quite  satisfactory  is  the  Edison- 
Lalande  cell:  this  is,  however,  expensive  in  the 
large  sizes  necessary  for  coi)  work. 

61 


Approximate  Specifications  for  Spark  Coils. 


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COILS  FOR  VARIOUS  LENGTHS  OF  SPARK, 


63 


TABLE  II. 

Wire  Gauges  Compared  in  Decimal  Parts  of  an  Inch. 


Number. 

American  or 

Brown  & Sharpe 

Gauge. 

Roebling’s  and 
Washburn  & Moen’s 
Gauge. 

E irmingham  or 
Stub’s  Wire  Gauge. 

British  Imperial 

Standard. 

English  Standard 

Gauge. 

Old  English  or 

London  Wire  Gauge 

0 

.32486 

.307 

.34 

.324 

.34 

2 

.25763 

.263 

.284 

.276 

.284 

4 

.20431 

.225 

.238 

.232 

.238 

6 

.16202 

.192 

.203 

.192 

.203 

8 

. 12849 

.162 

.165 

.16 

.165 

10 

.10189 

.135 

.134 

.128 

.134 

12 

.08081 

.105 

.109 

.104 

.109 

14 

.06408 

.08 

.083 

.08 

.083 

13 

. 05082 

.063 

.065 

.064 

.065 

18 

.0403 

.047 

.049 

.048 

.049 

20 

.03196 

.035 

.035 

.036 

.035 

22 

.02535 

.028 

.028 

.028 

.0315 

24 

.0201 

.023 

.022 

.022 

.025 

26 

.01594 

.018 

.018 

.018 

.0205 

28 

.01264 

.016 

.014 

.0148 

.0165 

30 

.01002 

.014 

.012 

.0124 

.01375 

32 

.00795 

.013 

.009 

.0108 

.01125 

34 

.0063 

.01 

.007 

.0092 

.0095 

36 

.005 

.009 

.005 

.0076 

.0075 

CHAPTER  VIII. 

Experiments  with  the  Induction  Coil. 

A book  dealing  with  the  subject  of  induction 
coils  would  hardly  be  complete  without  some 
reference  to  the  experiments  which  may  be  per- 
formed with  any  sparking  coil.  Those  in  which 
a shocking  coil  is  employed  are  of  too  obvious  a 
nature  to  require  description,  but  it  is  worth 
while  reminding  readers  that  many  people  can- 
not safely  bear  the  most  moderate  electrical 
shocks,  so  that  great  care  should  be  exercised 
when  experimenting  with  an  ordinary  medical 
coil.  It  need  hardly  be  said  that  experiments 
with  sparking  coils  should  be  conducted  with  the 
greatest  possible  care.  There  should  be  no  hurry; 
wires  should  all  be  connected  up  before  the  current 
is  switched  on,  and  no  one  but  the  operators 
should  be  allowed  to  touch  any  part  of  the  appa- 
ratus. 

For  the  best  experiments  a coil  capable  of  pro- 
ducing at  least  J-in.  sparks  is  desirable.  With 
this,  deflagration,  vacuum  tube,  and  wireless  tele- 
graphy experiments  can  be  carried  out.  For  X-ray 
work  or  radiography,  a 4-in.  spark  is  practically 
essential ; and  if  the  work  is  to  be  on  a professional 
or  semi-professional  scale,  a 6-in.  spark  is  the 
least  the  coil  should  give. 

64 


EXPERIMENTS  WITH  THE  INDUCTION  COIL.  65 


Begin  by  observing  the  character  of  the  sec- 
ondary spark.  If  two  points,  one  connected  to 
each  terminal  of  the  secondary,  are  made  to  ap- 
proach from  beyond  sparking  distance,  nothing 
will  be  observed  in  daylight  until  such  a position 
is  reached  that  the  longest  spark  can  bridge  the 
space.  It  will  be  seen  that  this  spark  is  of  a very 
beautiful  violet  color,  tending  to  red  at  one  end. 
The  character  of  the  spark  depends  upon  the 
distance  it  is  made  to  traverse;  at  its  greatest 
length  it  is  thin,  blue,  and  snapping,  and  takes 
a crooked  course;  when  shortened,  it  assumes  a 
flaming  appearance,  especially  at  one  pole,  goes 
in  a straight  line,  and  has  a more  rushing  sound. 
The  short  thick  spark  is  the  ‘ calorific  spark  ’ 
and  it  has  the  power  of  deflagrating  explosive 
mixtures. 

Experiments  should  be  made  on  the  character 
of  the  discharge  when  different  metals  are  used 
for  the  electrodes,  as  for  instance,  fine  iron  wire. 
In  this  case,  when  the  spark  begins  to  assume  the 
calorific  character,  one  of  the  wires  will  be  found 
t j grow  hot  and  ultimately  to  melt  off  at  the  end. 
This  is  the  negative  pole  of  the  secondary,  and 
the  effect  appears  to  be  due  to  a kind  of  bom- 
bardment which  takes  place  from  the  positive 
pole. 

For  ordinary  sparking  experiments  the  negative 
electrode  should  terminate  in  a fairly  large  brass 
disc,  the  positive  being  a needle  point  adjustable 
as  to  distance,  but  moving  opposite  the  centre 


66 


INDUCTION  COILS. 


of  the  disc.  By  this  means  the  longest  sparks 
can  be  obtained.  It  will  be  noticed  that  the  sparks 
constantly  strike  fresh  places,  and  they  almost 
invariably  traverse  very  crooked  paths  through 
the  air.  In  this  respect  they  are  like  lightning  flashes 
— 4 small  editions  ’ of  which  they  really  are,  as  a 
matter  of  fact.  These  crooked  paths  represent 
‘ lines  of  least  resistance,’  the  electric  current, 
no  doubt,  finding  irregularly  distributed  particles 
of  conducting  matter  floating  in  the  atmosphere. 

If  the  space  between  the  electrodes  be  increased 
beyond  sparking  distance,  sparks  can  be  made  to 
pass  by  holding  a lighted  spirit-lamp  so  that  the 
heated  gases  from  it  rise  up  between  the  poles. 
Similarly,  the  electrodes  being  adjusted  so  as  to 
bear  on  a glass  plate  even  twice  the  usual  maxi- 
mum distance  apart,  a spark  can  be  made  to  pass 
by  breathing  on  the  glass. 

Keeping  the  electrodes  in  the  position  last  in- 
dicated, scatter  some  fine  metal  filings  on  the 
glass.  Sparks  at  once  scintillate  in  a beautiful 
way  over  the  intervening  space,  leaping  from  one 
particle  to  another,  and  diverging  very  considerably 
from  a straight  path.  Repeat  this  experiment, 
but  using  finely  scraped  ‘ biacklead  ’ from  an 
ordinary  pencil.  This  gives  slightly  different 
effects,  and  a further  experiment  is  to  approach 
the  electrodes  closely  amongst  the  carbon  particles, 
which  then  begin  to  glow  vividly  like  a tiny  arc- 
lamp.  Next  put  a considerable  quantity  of  this 
powdered  carbon  on  the  plate  and  approach  the 


EXPERIMENTS  WITH  THE  INDUCTION  COIL.  67 


poles  to  the  little  heap ; it  will  be  seen  that  a clear 
space  is  made  round  the  positive  pole,  the  par- 
ticles being  apparently  blown  from  the  elec- 
trode. 

All  phenomena  of  sparking  are  very  much  more 
beautiful  when  the  experiments  are  conducted  in 
the  dark.  It  will  then  be  obvious  that  a great  deal 
has  been  lost  in  the  previous  experiments,  and 
glowing  brush-like  but  silent  discharges  will  be 
found  on  the  electrodes  when  these  are  separated 
quite  beyond  sparking  distance.  All  this  time 
the  characteristic  odor  of  ozone  will  be  noticeable 
even  when  a small  coil  is  being  worked.  This 
becomes  marked  when  a large  coil  is  operated, 
and  too,  much  of  this  very  energetic  form  of 
oxygen  should  not  be  inhaled. 

A darkened  room  is  the  proper  place  to  exhibit 
the  beautiful  experiments  which  can  be  performed 
with  vacuum  or  Geissler  tubes.  These  are  too 
varied  in  number  and  character  to  be  dealt  with 
here  at  any  length,  and  the  earnest  experimenter 
should  study  the  subject  carefully,  especially  if  an 
insight  into  the  marvels  of  X-ray  and  other  radio- 
graphic  work  have  attractions  for  him.  With 
the  smallest  sparking  coil  even,  very  beautitui 
experiments  can  be  made  with  small  vacuum 
tubes,  though  the  simplest  of  these  is  an  ordinary 
electric  incandescence.  It  does  not  matter  if  this 
is  a ‘burnt-out  ’ specimen,  so  long  as  the  vacuum 
is  perfect.  Hold  the  lamp  by  the  top  of  the  glass 
bulb  and  present  the  capped  end  to  either  electrode 


68 


INDUCTION  COILS. 


of  the  coil,  when  the  space  inside  the  lamp  will 
glow  with  an  indescribable  bluish  light,  which 
appears  even  when  the  lamp  is  held  several 
inches  away  from  any  part  of  the  coil.  Various 
rotating  devices  are  obtainable,  by  means  of  which 
a group  of  vacuum  tubes  can  be  spun  round  while 
glowing,  and  most  delightful  combinations  can 
be  made  in  that  way. 

Wireless  telegraphy  and  radiography  (or  X-rays) 
are  two  of  the  most  modern  instances  involving 
the  use  of  induction  coils.  Since,  however,  in  these 
matters  the  coil  and  its  sparking  properties 
are  but  means  to  other  ends,  and  as  very  long 
explanations  would  be  absolutely  necessary,  they 
cannot  be  dealt  with  here.  Suffice  it  to  say 
that  coils  as  described  in  this  book  are  quite  the 
proper  apparatus  for  experiments  in  these  direc- 
tions. 

Reference  may  be  made  in  conclusion  to  the  dis- 
continuous nature  of  the  secondary  spark.  Many 
people  suppose  that  the  bright  flash  is  uninter- 
rupted, but  that  this  is  not  the  case  can  be  shown  by 
a very  simple  yet  striking  piece  of  apparatus.  A 
large  disc  of  white  card — say  not  less  than  18  in. 
diameter — is  marked  out  and  blackened  as  in 
Fig.  35.  The  various  rows  of  squares,  circles,  etc., 
contain  each  a different  number  of  these  spaces. 
This  disc  is  mounted  by  being  nailed  to  a wooden 
boss  fixed  on  a spindle.  The  spindle  is  carried 
by  a simple  wooden  frame,  and  has  on  its  further 
end  a small  grooved  wheel.  Below  is  another 


EXPERIMENTS  WITH  THE  INDUCTION  COIL.  69 


spindle  on  which  is  fixed  a much  larger  pulley 
wheel,  in  which  again  is  fastened  a crank  handle 
to  turn  it.  A leather  bootlace  forms  a driving 
belt.  When  this  apparatus  is  worked  in  a dark- 
ened room  and  the  coil  is  set  in  operation,  the 


Fig.  35. 


curious  effect  will  be  observed  that  some  of  the 
rows  of  circles,  etc.,  appear  to  travel  in  one,  and 
others  in  the  other,  direction.  This  is  due  to  the 
intermittent  and  instantaneous  lighting  up  of  the 
secondary  spark.  As  the  disc  is  driven  faster  or 
slower,  some  of  the  moving  circles  run  slower, 


70 


INDUCTION  COILS. 


stop,  and  reverse  their  apparent  direction.  This 
experiment  is  an  extremely  interesting  one,  and 
usually  takes  well  even  amongst  the  variety  of 
other  striking  effects  obtainable  from  a good  in- 
duction coil. 


Note. — For  a more  complete  description  of  the 
construction  of  induction  coils,  see  Norrie. 

Also , Norrie  on  “Experimenting  with  Induction 
Coils.” 

Medical  Coils,  see  Allsop. 


■ 

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8 — Engineering  Notes.  Tables.  Index.  290  pages,  203  il- 
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